In a line conversion structure, a slot line includes a slot ground conductor connected to a ground layer with a through conductor that passes through the dielectric layer, a slot signal conductor, and a slot disposed between the slot ground conductor and the slot signal conductor. A signal conductor of a microstrip line is orthogonal to the slot ground conductor and the slot, with a gap between the signal conductor and the slot ground conductor, and an end of the signal conductor is connected to the slot signal conductor, and a length L of a portion of the slot ground conductor, the portion being parallel to the signal conductor with the gap, is less than or equal to 0.25 times a wavelength of a signal transmitted through the microstrip line.